/Engine/Source/Runtime/GeometryCore/Public/DynamicMesh/DynamicMeshOverlay.h
Goes through elements and frees any whose reference counts indicate that they are not being used. This is usually not necessary since most operations that remove references will go ahead and do this, but it may be used, for instance, after SetTriangle is called with bAllowElementFreeing set to false.
void FreeUnusedElements ( const [TSet](API\Runtime\Core\Containers\TSet)< int > * ElementsToCheck )